The present invention relates to a method of, and system for, transmitting messages and also to a primary station and a secondary station for use in the system. An example of such a system is an answer back paging system and for convenience reference will be made to an answer back paging system but it is to be understood that the teachings of the present invention can be applied to other 2-way messaging systems.
Answer back paging systems have been disclosed for example in WO96/14716. In an elementary form such a system requires a paging network controller (PNC) to arage for a message to be transmitted to a predetermined addressee. The addressee on receivig a call is able transmit a simple reply by way of a low power transmitter incorporated into the pager. The above mentioned Patent Specification discloses a system in which a series of messages are transmitted to respective addressees and the PNC then transmits invitations for the addressees to transmit their replies substantially simultaneously as pseudo random data sequences which are de-spread at the PNC and the replies forwarded to the respective party requiring the reply. In order to avoid having to apply power control techniques in the pagers to ensure that replies at any one instant are received by the PNC at comparable power levels, the PNC transmits its invitations at stepwise increasing (or decreasing) power levels and only those pagers just able to receive a respective one of the invitations transmit their replies. A feature of this known method is that relatively strict control of the received power is necessary.
PCT Patent Application IB97/00492 discloses a variant of the above mentioned method which for convenience of description will be referred to as progressive elimination. In this technique the PNC transmits a series of messages on a downlink to individually addressed pagers. The PNC then transmits a control signal on the down-link inviting those pagers wishing to make an up-link transmission, for example a reply to a message, request for service or a registration request, to transmit them substantially simultaneously as pseudo-random data sequences. The PNC analyses those of the up-link transmissions which are intelligible and then repeats the invitation which includes acknowledgements of those up-link transmissions that have been analysed successfully, consequently only those pagers who had responded the first time but had not received an acknowledgement need retry.
Essentially these known techniques require each pager to store a number of pseudo-random data sequences, each one specific to a particular type of response. However this leads to some inflexibility when processing group messages, responding individually to several stored messages and fragmented messages.